1. Field of the Invention
The invention relates to an image display device including an image display cell and a polarizing plate placed on a viewer side of the image display cell.
2. Description of the Related Art
A liquid crystal display (LCD) has a liquid crystal panel that includes a liquid crystal cell and a polarizing plate placed on at least a viewer side of the liquid crystal cell so that it enables display by controlling an amount of transmission of light emitted from a light source or reflected light resulting from external light incident on the liquid crystal cell. As LCDs have been rapidly developed, their functions and applications tend to be diversified, and they are required to withstand more harsh environments during use. For example, LCDs for mobile applications such as cellular phones and for vehicle applications such as car navigation systems are required to have durability even to more harsh environments during use. In addition, large-sized LCDs such as LCD televisions are required to have durability to higher temperature, because as their size and brightness increase, an amount of heat from the light source increases, so that the LCDs themselves tend to have high temperature. Thus, polarizing plates for use in LCDs are also required to have less changeable properties under harsh environments such as high temperature and high humidity, namely, to have high durability.
An organic electroluminescent (EL) display device has an organic EL cell including an organic light-emitting layer and enables display by controlling an amount of luminescence from the organic EL cell. Since the organic light-emitting layer has a very small thickness such as about 10 nm, external light is reflected from a metal electrode to a viewer side, so that the display surface of the organic EL display device looks like a mirror surface, when viewed from outside. To block such specular reflection of external light, the technical method used includes placing a circularly polarizing plate including a laminate of a polarizing plate and a quarter wavelength plate on the viewer side of the organic light-emitting layer. Organic EL display devices are also required to have durability even to more harsh environments during use. Therefore, the polarizing plate placed on the viewer side is also required to have less changeable properties under harsh environments such as high temperature and high humidity, namely, to have high durability.
In general, a polarizing plate has a structure in which a polarizer is disposed between two protective films, and triacetylcellulose (TAC) is widely used for the protective films. On the other hand, in view of the durability described above, it is proposed that a film with a high level of mechanical properties, chemical resistance and water-barrier properties, such as a polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) film be used as a polarizer-protective-film (see for example Japanese Patent Application Laid-Open (JP-A) No. 08-271733).
A polyester film such as a PET or PEN film is highly stretched and crystallized and therefore has a high level of mechanical properties and other properties as mentioned above. However, the polyester film has high birefringence in the in-plane direction and the thickness direction, because its intrinsic birefringence is high and is highly stretched. Therefore, when a film made of a high-birefringence material, such as a polyester film is placed as a polarizer-protective-film between a polarizer and an image display cell, polarization is distorted between the polarizer and the image display cell due to an effect of the birefringence, so that visibility may tend to be significantly reduced. From this point of view, such a polyester film is generally used as a protective film for the principal surface of the polarizer on the side not facing the image display cell.
Such a film provided to protect the principal surface of the polarizer on the side not facing the image display cell does not change the polarization between the polarizer and the image display cell. Therefore, the effect of the birefringence on the display properties is small, and the film is not necessarily required to have optical isotropy or birefringence uniformity. However, there is a problem in which when a polyester film is used as a protective film placed on the viewer side principal surface of the polarizer in a polarizing plate placed on the viewer side of an image display cell, iridescent unevenness (rainbow like unevenness) occurs to degrade visibility. Particularly, as the brightness and color purity of image display devices have increased in recent years, such iridescent unevenness becomes more likely to be visible, which becomes a hindrance to the use of a polyester film as a polarizer-protective-film.
From this point of view, it is proposed that a light scattering layer should be provided on the surface of a polarizer-protective-film so that mixing of optical paths, namely, mixing of colors can be made to make iridescent unevenness less visible. For example, JP-A No. 2008-3541 discloses that when a light scattering layer is provided on the surface of a polyester film with an in-plane retardation of 500 nm or more to form a polarizer-protective-film, such iridescent unevenness can be reduced. Such a method can make iridescent unevenness less visible but does not prevent iridescent unevenness itself, so that sufficient visibility cannot be obtained in some cases. As the brightness and color purity of image display devices further increase, the occurrence of iridescent unevenness becomes more significant. It is therefore considered that it will be difficult to ensure sufficient visibility by such a color mixing method for canceling iridescent unevenness.